This invention relates generally to fume hoods, and in particular to energy-efficient laboratory fume hoods. More specifically, the invention relates to laboratory fume hoods which use low flow rates and further relates to structural features which facilitate containment of contaminants in a fume hood.
A fume hood may be generally described as a ventilated enclosed workspace intended to capture, contain, and exhaust fumes, vapors, and particulate matter generated inside the enclosure. The purpose of a fume hood is to draw fumes and other airborne matter generated within a work chamber away from a worker, so that inhalation of contaminants is minimized. The concentration of contaminants to which a worker is exposed should be kept as low as possible and should never exceed a safety threshold limit value. Such safety thresholds and other factors relating to testing and performance of laboratory fume hoods are prescribed by government and industry standards by organizations, such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) of Atlanta, Ga., for example, ANSI/ASHRAE 110-1995. ASHAAE Standard, xe2x80x9cMethod of Testing Performance of Laboratory Fume Hoods.xe2x80x9d This and all other documents cited in this application are incorporated herein by reference for all purposes.
FIG. 1 shows a cross-sectional side view of a conventional fume hood. The hood 100 includes a work chamber 102, bounded by walls 103 and a front open face 105 which may be covered partially or completely by a moveable sash 114. The hood may be supported by a base 104. In many designs, the base contains cabinets for storage of solvents and other materials used in the hood""s work chamber 102.
While hood sizes vary considerably, a typical conventional fume hood is about 4 to 10 feet wide with a sash opening of between about 26 and 34 inches, and a standard interior vertical size of about 52 inches. The hood""s sidewalls 103 typically have considerable thickness because they contain mechanical and electrical services for the hood. Again, while dimensions of fume hoods greatly vary, the depth of a typical fume hood ranges from about 32 to about 37 inches. A typical conventional hood design includes an air foil 106 at the bottom front of the work chamber 102 and a baffle 108 at the rear of the work chamber 102. The depth of the work chamber 102 between these two features 106 and 108 is typically approximately 21 inches.
The opening in the front of the fume hood 100 which provides access to the work chamber 102 by a worker, is referred to as the face of the fume hood. In some conventional fume hood designs, referred to as open-faced hoods, the face area of the hood is fixed. In other designs, such as that depicted in FIG. 1, the moveable sash 114 provides the ability to alter the face area of the hood 100. Sashes come in either vertical or horizontal arrangements, with the vertical design typically being preferred since it can provide a full open face area.
Other elements of conventional fume hoods illustrated in FIG. 1 include an air bypass area 116 above the sash in the top front of the fume hood 100 which provides an additional path for ambient air to enter the work chamber 102. The bypass 116 provides sufficient air flow to dilute contaminants in the hood, and to avoid air whistling when the sash 114 is closed. Air is exhausted from the fume hood through an exhaust system equipped with a fan (not shown) which draws air into the fume hood""s work chamber 102, through the baffle 108, and into ducting 118 outside the work chamber 102 of the fume hood 100 for exhaustion from the building. The top wall of the fume hood is also typically equipped with a light fixture 120 to illuminate the work chamber 102. The back baffle 108 typically includes two or three horizontally disposed slots to direct air flow within the work chamber 102. Further details regarding the design and construction of conventional laboratory fume hoods may be found in Sanders G. T., 1993. Laboratory Fume Hoods, A User""s Manual. John Wiley and Sons, Inc.
Containment of contaminants in many conventional fume hoods is based on the principal of supplying an abundant amount of air into the face of the hood and withdrawing this air, along with the contaminants, from the work chamber. As noted above, the face corresponds to the area below the sash (in the case of a vertical sash arrangement) at the front of the hood through which air enters the work chamber. This abundant amount of air is supplied at a high enough rate such that contaminants within the hood are prevented from moving against the incoming air entering the face of the hood. Under conventional principles, air flow is typically increased to improve containment of contaminants within the work chamber.
An important factor in a conventional fume hood""s ability to contain contaminants is its face velocity. The face velocity of a fume hood is determined by its exhaust and its open face area. Recommendations for face velocity of conventional fume hoods range from 75 feet per minute (fpm) for materials of low toxicity (Class C: TLV greater than 500 ppm) to 130 fpm for extremely toxic or hazardous materials (Class A: TLV less than 10 ppm). Cooper, E. C., 1994. Laboratory Design Handbook, CRC Press. In general, industrial hygienists recommend face velocities in the range of 100 fpm plus or minus 10 fpm for containment of contaminants by conventional hoods with open sashes.
Face velocities at these speeds typically produce turbulent air flow conditions within the hood. As a result, unpredictable and inconsistent air flow patterns, such as vortices near exhaust outlets and near the face of the hood, often occur. The unpredictability of turbulent air flow conditions within the hood may result in reversal of flow near the face of the hood despite the high velocity of incoming air, causing contaminants to spill from the hood""s work chamber into the surrounding laboratory space. Turbulent air flow within the hood also increases mixing between the fresh air and other airborne contaminants generated within the work chamber.
The abundant amount of air supply provided to the hood and turbulent air flow conditions formed therein are often compounded by conventional fume hood design. FIG. 2 shows a cross-sectional side view of a conventional fume hood design, such as that illustrated in FIG. 1, further illustrating ideal air flow through such a conventional hood. Air is shown entering the hood 200 from the surrounding laboratory space 201 by arrows 202. The air flows through the open face 203 of the hood 200 defined by the fully open sash 206 and the air foil 208 into the work chamber 205. Inside the work chamber 205 the air is drawn towards slots 204 in the baffle 207 at the rear of the work chamber 205. In the particular design depicted in FIG. 2, the air flow generated by the slots establishes a vortex 210 in the upper region of the work chamber. If this vortex extends to or below the upper limit of the open face 203, the risk of spillage of airborne contaminants from the hood 200 is increased. Having passed through the baffle 207, the air is then exhausted through the exhaust system 212.
In addition to the hood design, the position of the worker with respect to the air flow direction may have a significant influence on the air flow patterns in the hood, and particularly in the face of the hood. Air flows surrounding a body standing in front of the hood create a region of low pressure downstream of the body. This region, which is deficient in momentum, is called the wake. It disturbs the directed air flow in the face of the hood, adding to any turbulence and may further result in reversal of flow causing contaminants to spill from the hood""s work chamber into the surrounding laboratory space.
As described above, the air source for conventional fume hoods is the ambient air in a laboratory in which the fume hood is located. The additional air which must be provided to a laboratory space by a building""s HVAC system to replace air exhausted by a fume hood is referred to as xe2x80x9cmake-up air.xe2x80x9d Since make-up air is supplied as part of the laboratory""s ambient air, it must be conditioned to the same degree if comfort and safety levels in the laboratory are to be maintained. As a result, laboratory buildings have very high energy intensities. Conditioning of the make-up air to be exhausted by fume hoods uses most of the energy beyond what is required for technical apparatus and lighting in laboratory environments. The high energy consumption caused by fume hood exhaust air flows is a result of both the need to condition make-up air and in conventional systems and to move it through a building""s air flow handling system. Thus, the abundant amount of air provided for the operation of conventional laboratory fume hoods results in a tremendous energy wastage.
Accordingly, alternative fume hood designs which reduce the amount of air required for operability, reduce energy consumption and provide containment of contaminants would be desirable.
To achieve the foregoing, the present invention provides a fume hood that offers an adequate containment of contaminants while reducing the amount of air exhausted from the hood. The fume hood includes a plurality of air supply outlets which provide fresh air, preferably having laminar flow, to the fume hood. The fume hood also includes an air exhaust which pulls air from the work chamber in a minimally turbulent manner. The push of the air supply outlets and the pull of the air exhaust form a push-pull system that provides a low velocity displacement flow which displaces the volume of gases currently present in the hood in a minimally turbulent and substantially consistent manner. As a result, inconsistent flow patterns associated with turbulent air supply and contaminant escape from the fume hood are minimized. The displacement flow fume hood in accordance with one embodiment of the present invention largely reduces the need to exhaust large amounts of air from the hood. It has been shown that exhaust air flow reductions of up to 70% are possible without a decrease in the hood""s containment performance.
The present invention includes a number of structural features which facilitate consistent and minimally turbulent flow within a fume hood. In one embodiment, the present invention includes a tapered wall on the top of the work chamber which facilitates flow towards an upper air outlet from the chamber and minimizes the formation of vortices near the top of the chamber. In another embodiment, the present invention includes an air supply within the work chamber to facilitate flow of gases in a minimally turbulent and substantially consistent manner.
In one aspect, the invention relates to a fume hood. The fume hood includes a partially enclosed work chamber having a front open face, a first top air source at the face of the work chamber, and a second top air source inside the face of the work chamber. The fume hood additionally includes a bottom air source at the face of the work chamber, and at least one air exhaust outlet from the work chamber.
In another aspect, the invention relates to a fume hood including a partially enclosed work chamber having a front open face and a top angled wall partially enclosing the work chamber. The fume hood also includes a top air source at the face of the work chamber, a bottom air source at the face of the work chamber, and at least one air exhaust outlet from the work chamber.
In yet another aspect, invention relates to a displacement flow fume hood including a partially enclosed work chamber having a front open face, the front open face having a front open face area. The displacement flow fume hood also includes a first top air source at the face of the work chamber, a second top air source, and a bottom air source at the face of the work chamber. The displacement flow fume hood further includes at least one air exhaust outlet associated with a work chamber outlet, the work chamber outlet having an outlet area, wherein the work chamber has a cross section area along a line of air flow between the open face and the work chamber outlet which is greater than the front open face area and which is greater than the chamber outlet area.
In another aspect, the invention relates to a method of preventing airborne contaminants from escaping through the face of a fume hood, the fume hood having a partially enclosed work chamber having a front open face. The method comprising supplying an air flow to said face through a plurality of air sources including a first top air source at the face of the work chamber, a second top air source inside the work chamber, and a bottom air source at the face of the work chamber.
These and other features and advantages of the present invention are described below with reference to the drawings.